1. Technical Field
The present invention relates to an apparatus and method for improving the sensitivity of a flow rate device measuring the flow rate of a fluid in a hydrocarbon well. More particularly, but not by way of limitation, the present invention relates to a flow restrictor insert that can be installed in a restriction of a differential pressure measurement device to increase the velocity of the fluid as it passes through the restriction and can be permanently removed by destruction while maintaining the operability of the flow rate device albeit at a lower sensitivity.
2. Background Art
Flow rate devices, or flow meters, are widely used in the oil and gas, and petrochemical industries. The ability of the oil and gas industry to optimize production of a reservoir relies on the possibility of evaluating the well effluent (i.e. oil, gas and water), at regular intervals, in terms of quantity (flow rate) and of composition (the proportions of the various phases).
There are a number of current systems available to measure the fluid flow rate flowing from a well, but few, if any, of them can be constructed “in-line,” to allow reliable, unobtrusive measurements to be taken within the wellbore at a location near the reservoir. Most flow rate measurement systems are installed at the surface of the well-site and are adapted to interpret, or measure, the flow rate of the well effluent after it has traveled from the reservoir to the surface. Furthermore, in certain systems, an in-line flow rate device measures the flow rate of a fluid in a hydrocarbon well using a differential pressure measurement device, such as a Venturi or orifice plate.
Numerous proposals based on Venturi-type flow rate devices have been made to evaluate the wellbore fluid. For example, U.S. Pat. Nos. 4,757,709 and 5,736,650, the entire contents of which are expressly incorporated herein by reference, provide that differential pressure measurement can allow a determination of fluid flow rate when combined with various physical properties of the fluid, such as density. In such proposals, it is suggested that the flow rate device can be provided with a differential pressure measurement device that measures the difference in pressure of the fluid as it flows past Venturi ports prior to and within the Venturi's restriction.
As another example, U.S. Pat. No. 7,114,401 issued on Oct. 3, 2006 is directed towards a flow measurement system using, for example, a Venturi for steady-state and transient flow rate measurement, particularly suited for the drilling domain. The '401 reference, however, measures differential pressure by measuring the radial deformation, changes in the hoop strain/stress, of a sleeve upstream and downstream of a Venturi-type section.
In a Venturi flow rate device the differential pressure (ΔP) between the Venturi ports is mainly controlled by the flow velocity in the restriction, and that for a given rate, the higher the velocity, the more accurate will be the measurement. In other words, flow rate devices produce more accurate measurements when a greater pressure differential is produced, that is by going from a larger diameter to a smaller diameter. The smaller the restriction's diameter, the better the quality of the rate measurement.
As an example, U.S. Pat. No. 6,993,979 issued Feb. 7, 2006 relates to an insert device for narrowing a throat of a Venturi-based multiphase flow meter. As mentioned before, however, the flow meter of the '979 is particularly suited for installation at the surface of the well-site. Moreover, the insert device is not particularly adapted to be permanently removed by destruction so as to not interfere with wellbore operations.
Industry practice dictates that for a Drill Stem Testing operation the throat restriction should not go below 2.25 inches in order to allow for well intervention (i.e., logging, fishing, firing using a drop bar, or the like). At this throat size, the differential pressure output is less accurate at low flow rates (e.g., <2,000 bbl/d). Further, restrictions below 2.25 inches can either cause blockage or limit the selection of tools that may be used when deploying tools down/up a production well.
It is therefore desirable to provide a method and apparatus for increasing the velocity of the fluid as it passes through the restriction of the Venturi while preserving the ability to allow for well intervention. It is also desirable to measure the flow rate of the well effluent at a location near the reservoir rather than at surface, so as to disregard variables such as phase changes and gas expansion that occur as the well effluent travels across pressure and temperature differences between the reservoir and surface.